1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same, and in particular, to a zoom lens and an image pickup apparatus suited to a digital camera, a video camera, a TV camera, a monitoring camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, an imaging lens used in an image pickup apparatus such as a TV camera, a digital camera, and a video camera is required to be a zoom lens having high optical performance over the entire zoom range. Also, the imaging lens is desired to have a high zoom ratio to enlarge an imaging area. To obtain high optical performance over the entire zoom range, it is important to appropriately correct chromatic aberration in addition to correcting monochromatic aberrations such as spherical aberration and coma aberration.
In particular, for a zoom lens whose focal length on the telephoto side is long and which has a high zoom ratio, it is important to appropriately correct secondary spectrum as correction of chromatic aberration in addition to primary achromatism. As a zoom lens having a high zoom ratio, a so-called positive lead type zoom lens is known in which a lens unit having a positive refractive power is disposed nearest to the object. As a positive lead type zoom lens, a zoom lens is known in which an anomalous dispersion material is used as a material of lenses of the first lens unit and the chromatic aberration (secondary spectrum) is appropriately corrected (U.S. Pat. No. 6,594,087).
Also, a zoom lens is known in which lenses formed of an anomalous dispersion material are used in the first lens unit in a zoom lens made up of five lens units respectively having positive, negative, positive, negative, and positive refractive powers in order from the object side (U.S. Pat. No. 7,304,805).
Also, a zoom lens is known in which the chromatic aberration is corrected by using lenses formed of a material of low dispersion having a high partial dispersion ratio in the second lens unit in a zoom lens made up of five lens units respectively having positive, negative, positive, negative, and positive refractive powers in order from the object side (U.S. Pat. No. 7,177,092).
On the other hand, a zoom lens is known in which lenses formed of a material of high dispersion having a high partial dispersion ratio are used in the second lens unit in a negative lead type zoom lens made up of a first to a third lens units respectively having negative, positive, and positive refractive powers in order from the object side (Japanese Patent Laid-Open No. 2008-233161).
It is relatively easy to realize a high zoom ratio by using a positive lead type zoom lens. However, the secondary spectrum of axial chromatic aberration tends to be large in a zoom area on the telephoto side. In a positive lead type zoom lens, to obtain high optical performance over the entire zoom range even when the zoom lens has a high zoom ratio, it is important to reduce chromatic aberration, in particular, to reduce the secondary spectrum in addition to the primary chromatic aberration.
To reduce the primary chromatic aberration and the secondary spectrum, it is effective to use a lens formed of a material of low dispersion having anomalous dispersion property in an appropriate position in a zoom lens. Regarding the chromatic aberration, it is important to optimize refractive powers of the lens units including lenses formed of materials described above on the basis of material characteristics (Abbe number and partial dispersion ratio). For example, in a zoom lens having a high zoom ratio, the chromatic aberration of magnification on the wide-angle side and the chromatic aberration of magnification on the telephoto side tend to be generated in the opposite directions. In this case, it is desirable that a lens formed of a lens material having an excellent effect for correcting the chromatic aberration is disposed in the second lens unit where the incidence height of the principal ray largely varies.
For example, in the positive lead type zoom lens described above, it is important to appropriately set materials of lenses included in the first lens unit having a positive refractive power and the second lens unit having a negative refractive power, and if the materials of these lenses are inappropriate, it is difficult to reduce the secondary chromatic aberration of magnification. As a result, it is difficult to obtain high optical performance over the entire zoom range.
The present invention provides a zoom lens of positive lead type, in which the secondary chromatic aberration of magnification can be reduced and high optical performance can be obtained over the entire zoom range while achieving a high zoom ratio, and an image pickup apparatus having the zoom lens.